Heat-shrinkable tubes are used in various applications, for example, for the covering of joints of electric wires or cables, the terminal processing of wires in various equipments, or for the prevention of corrosion of steel pipes. When heat-shrinkable tubes are used for the connection or terminal processing of wires in equipments, or for the covering of condensers, they must excellent transparency so that the connection can be seen or so that the type of equipment used can be determined easily. Furthermore, when they are used in such applications, they must meet the requirements for flame-retardant properties as specified in UL Standards. Heat-shrinkable tubes for use in such applications, therefore, are required to have high flame retardant properties.
Conventional heat shrinkable tubes composed of polyvinylidene fluoride (U.S. Pat. No. 3,580,829) and of polyesters, e.g., Mylar (U.S. Pat. No. 3,040,385) whose cristallinity is high have flexibility and transparency poorer than those composed of a polyvinyl chloride (PVC).
Thus, in the field including the above-described applications, heat-shrinkable tubes made of compositions containing PVC having excellent transparency have heretofore been used. These transparent PVC tubes usually contain plasticizers to improve flexibility because they have poor flexibility although they have excellent transparency. These plasticizers, however, ooze out, causing problems such as contamination of the surface of equipment, or transferring into insulated wires which are covered with the tube. Thus, heat-shrinkable tubes not containing a plasticizer, i.e., of so-called unplasticized type, have been desired. However, there has not yet been discovered a satisfactory heat-shrinkable tube of the unplasticized type. For example, a polyethylene (PE) resin which is a typical example of unplasticized resins is inferior in flame-retardant properties and transparency, and an ethylene-vinyl acetate copolymer (EVA) is inflammable although it has excellent transparency. However, addition of a flame retardant to EVA results in the loss of transparency.